It is known in the art that there are two types of solid electrolytes are used in a high energy density battery, one is made from inorganic acid salts and the other one is made from organic polymers. The inorganic acid salt eletrolytes have a superior conductivity, but also suffer a lower decomposition electrical potential and a higher interface resistance between the electrode and the electrolyte in comparision with the organic polymer electrolytes.
The majority of polymer electrolyte systems reported in the literature to date have largely based on poly(ethylene oxide), PEO, incorporating an alkali metal salt. Recently, some studies were focused on the complexes formed by alkali metal salts and comb-like polymers containing oligo(oxyethylene) side chains. Interest in these comb-like polymers for the preparation of polymer electrolytes, comes from the high segmental mobility of side chains and the low glass transition temperature exhibited by these polymers. Several articles have reported on the conductivity of solid complexes of Li-salts and comb-like polysiloxanes with oligo(oxythylene) side groups, such as an article by
Hall, et al. entitled "Ion conductivity in polysiloxane comb polymers with ethylene glycol teeth" Polym. Commun., 27, 98 (1986); another article by Cowie, et al. entitled "Ionic conductivity in poly(di-poly(propylene glycol)itaconate)-salt mixtures" Polymer, 28, 627 (1987), and another article by Fish, et al. entitled "Conductivity of solid complexes of lithium perchlorate with poly{[w-methoxyhexa(oxyethylene)ethoxy]methylsiloxane} Makromol. Chem., Rapid Commun., 7, 115 (1986).
Since Ringsdorf, Finkelmann and Wendorff introduced the spacer concept in their articles entitled "Polyreactions in ordered system, 14. Model considerations and examples of enantiotropic liquid crystalline polymers" Makromol. Chem., 179, 273 (1978), and "Liquid crystalline polymers with biphenyl-moities as mesogenic group" Makromol. Chem., 179, 2541 (1978), a large variety of side-chain liquid crystalline polymers have been synthesized and characterized. However, the vast majority of these side-chain liquid crystalline polymers contain polymethylene units, i.e. --(CH.sub.2)n-- (n=1-14), as flexible spacers. So far, there are few examples of side-chain liquid crystalline polymers based on oxygen containing flexible spacers (i.e., oligooxyethylene). For more detailed information related to these side-chain liquid crystalline polymers, please refer to the following articles:
1. M. Engel, B. Hisgen, R. Keeler, W. Kreuder, B. Reck, H. Ringsdorf, H. W. Schmidt and P. Tschirner, "Synthesis, structure and properties of liquid crystalline polymers", Pure & Appl. Chem., 57, 1009 (1985). PA0 2. J. M. Rodriguez-Parada and V. Percec, "Poly(vinyl ether)s and poly(propenyl ether)s containing mesogenic groups: A new class of side-chain liquid-crystalline polymers", J. Polym. Sci., Polym. Chem. Ed., 24, 1362 (1986). PA0 3. V. Percec, J. M. Rodriguez-Parada, and C. Ericsson, "Synthesis and characterization of liquid crystalline poly(p-vinylbenyl ether)s", Polym. Bull., 17, 347 (1987). PA0 4. R. Duran and P. Gramain, "Synthesis and tacticity characterization of a novel series of liquid crystalline side chain polymers with oligo(ethylene oxide) spacers", Makromol. Chem., 188, 2001 (1987). PA0 5. R. Duran and C. Strazielle, "Molecular weights and solution properties of a series of side-chain liquid crystalline polymers with ethylene oxide spacers", Macromolecules, 20, 2853 (1987). PA0 6. R. Duran, P. Gramain, D. Guillon and A. Skoulios, "Novel liquid crystals from side-chain polymers with oligoethyleneoxide spacers", Mol. Cryst. Liq. Cryst., Lett., 3, 23 (1986). PA0 7. S. G. Kostromin, V. P. Shibaev and N. A. Plate, "Thermotropic liquid-crystalline polymers XXVI. Synthesis of comb-like polymers with oxygen containing spacers and a study of their pphase transstions", Liquid Crystals, 2, 195 (1987). PA0 8. C. Kim. and H. R. Allcock, "A liquid Crystalline poly(organophosphazene)", Macromolecules, 20, 1726 (1987).
Mr. Chain-Shu Hsu, who is one of the co-inventers of present invention, and V. Persec in their articles entitled "Synthesis and Characterization of Liquid Crystalline Polysiloxanes Containing Benzyl Ether Mesogens" J. Polym. Sci., Polym. Chem. Ed., 25, 2909 (1987), and "Synthesis and Characterization of Liquid Crystalline Polyacrylates and Polymethacrylates Containing Benzyl ether and Diphenyl Ethane Based Mesogens" J. Polym. Sci., Polym. Chem. Ed., 27, 453 (1989), disclose the synthesis and characterization of side-chain liquid crystalline polysiloxanes, polymethacrylates and polyacrylates containing benzyl ether and diphenyl ethane based mesogens. Because of the free rotation around the benzyl ether or ethylene bonds, these polymers represent the simplest class of liquid crystals containing mesogenic groups that present conformational isomerism. The phase behavior of these polymers showed that these kinds of mesogenic groups depress the tendency towards side chain crystallization behavior of the side-chain liquid crystalline polymers.
An object of the present invention is to provide ionic conducting polymer electrolytes which are prepared from alkali metal salts and side-chain liquid crystalline polysiloxanes containing oligooxyethylene spacers and benzyl ether based mesogenic groups. The polysiloxanes used in the present invention have very low glass transition temperatures, exhibit high mobility of the side chains at the mesophase, and form pseudocrown ether structure between the oligooxyethylene spacers, which in turn enhance the solubility of the alkali metal salts. As a result, the complexes formed by the polysiloxanes and alkali metal salts exhibit high ionic conductivities.